1. Field of the Invention
The present invention relates to a current mirror circuit and a semiconductor integrated circuit having the current mirror circuit, and more particularly, to a current mirror circuit using a FET (field effect transistor) capable of being driven at a low voltage.
2. Description of the Related Art
Conventionally, a current mirror circuit has been widely used for various circuits and is required to be capable of setting a mirror ratio with high accuracy. A circuit on which the current mirror circuit is mounted, on the other hand, tends to require a low driving voltage, and therefore a current mirror drivable with a low voltage is required.
A conventional current mirror circuit comprises, for example, a reference current source for supplying a reference current, an input circuit and an output circuit. For example, the input circuit includes a n-channel MOS transistor (nMOS transistor), and the output circuit includes a nMOS transistor. These transistors of the input circuit and output circuit constitute a current mirror fashion (current mirror circuit).
The transistor of the output circuit is saturated when the voltage applied to an output of the current mirror circuit is not lower than a voltage where an output current is substantially constant. Due to the channel modulation effect, however, an output impedance of the current mirror circuit is reduced. Specifically, the output current after saturation increases with the voltage applied to the output of the current mirror circuit, resulting in a reduced current accuracy.
Further, in a conventional cascode current mirror circuit, where a gate-grounded transistor is inserted for cascode connection thereby to realize a high-output impedance, a low source voltage cannot be used. Namely, the conventional cascode current mirror circuit requires a high saturation voltage, and therefore, fails to meet the recent requirement for a lower voltage.
Therefore, in the conventional current mirror circuits, it is difficult to secure a high current accuracy and a low source voltage at the same time.
The prior art and the problem points thereof will be described in detail later with reference to the accompanying drawings.